Electrical Impedance Tomography - Lung Imaging

Lung Imaging

EIT is useful for monitoring patient lungs because the air has a large conductivity contrast to the other tissues in the thorax. The most promising clinical application of lung EIT measurements is for Lung function monitoring of patients being treated with Mechanical ventilation. Such ventilation can often result in Ventilator-associated lung injury. EIT can resolve the changes in the distribution of lung volumes between dependent and non-dependent lung regions as ventilator parameters are changed. Thus, EIT measurements may be used to control the specific ventilator settings to maintain lung protective ventilation for each patient.

The above images are from the EIT group at Oxford Brookes University and depict an early attempt at three dimensional EIT imaging of the chest using the OXBACT3 EIT system. The reconstructed image is a time average and shows lungs as low conductivity regions. Although an accurate chest shape was used only a 2D reconstruction algorithm was used resulting in a distorted image. The results of a similar chest study have been published.

Most EIT studies have focused on regional lung function monitoring using the information determined by functional EIT (f-EIT). However absolute EIT (a-EIT) also has the potential to become a clinically useful tool for Lung imaging, as this approach would allow one to directly distinguish between lung conditions which result from regions with lower resistivity (e.g. hemothorax, pleural effusion, atelectasis and lung edema) and those with higher resistivity (e.g. pneumothorax, emphysema).

The reconstruction of absolute impedance images requires that the exact dimensions, the shape of the body and the precise location of the electrodes, be taken into account, as simplified assumptions would lead to major reconstruction artifacts. While initial studies assessing aspects of a-EIT have already been published, as of today this area of research has not yet reached the level of maturity which would make it suitable for clinical use. In contrast, functional EIT determines relative impedance changes that may be caused by either ventilation or changes of end-expiratory lung volume. These relative changes are referred to a baseline level, which is typically defined by the intra-thoracic impedance distribution at the end of expiration. Functional EIT images can be generated continuously, directly at the bedside. These attributes make regional lung function monitoring particularly useful whenever there is a need to improve oxygenation or CO2 elimination and when therapy changes are intended to achieve a more homogenous gas distribution in mechanically ventilated patients. EIT lung function imaging can resolve the changes in the regional distribution of lung volumes between e.g. dependent and non-dependent lung regions as ventilator parameters are changed. Thus, EIT measurements may be used to control the specific ventilator settings to maintain lung protective ventilation for each patient.